As known, EECs are used to electronically control operation of gas turbine engines based on selected control algorithms executed by the EEC. The EEC applies the control algorithms to sensed engine parameters received in the form of electrical signals to provide electrical control signals to electro-mechanical interfaces, such as position controlled actuators, which position various engine valves, vanes, etc. to regulate engine performance. To ensure reliability the EEC includes dual redundant control circuitry, i.e. primary and secondary control channels. The sensed engine parameter signals are also redundant. Each parameter has duplicate sensed signal inputs to each of the channels of the EEC. These sensed parameter signals include analog amplitude and analog frequency signal formats, as well as digital.
Digital electronic engine controls (DEEC) use digital signal processing to apply the control algorithms to the sensed parameter signals. For the analog sensed signal formats this requires analog-to-digital (A/D) conversion, all of which is well known. Since A/D conversion is sensitive to spurious noise associated with the analog signal, such as electromagnetic interference (EMI), it is also known to provide the DEEC with input selection logic. This logic selects the best signal from among the two actual sensed parameter signals and an idealized, synthesized parameter signal which acts as referee.
The most critical gas turbine engine parameters are rotor speed; both N.sub.1, N.sub.2. Each are sensed as frequency signals; typically the position of a ring gear mounted to the shaft is sensed by duplicate magnetic pick-ups which provide redundant cyclic signals (N.sub.A, N.sub.B) whose frequency is indicative of angular velocity. The DEEC converts the frequency signals to digital signals representative of engine RPM. However, these frequency signals are susceptible to lightning induced transients, or EMI, which results in erroneous frequency-to-digital conversion which affect RPM accuracy and cause unwantd control response. Although electrical shielding on the sensor lines may be used, the cost and weight may be prohibitive due to the number of sensors and the wire lengths.